This invention relates to methods and apparatus for rapid and accurate advancement of print media.
Ink-jet printers of interest here include at least one print cartridge that contains ink within a reservoir. The reservoir is connected to a printhead that is mounted to the body of the cartridge. The printhead is controlled for ejecting minute drops of ink from the printhead to a sheet of print medium, such as paper, that is advanced through the printer.
The printer includes a carriage for holding the print cartridge. The carriage is scanned across the width of the paper, and the ejection of the drops onto the paper is controlled to form a swath of an image with each scan. The height of the printed swath (as measured in the direction the media is advanced) is fixed for a particular printhead.
Between carriage scans, the media is advanced so that the next swath of the image may be printed. In most cases, the base of the just-printed swath must be precisely aligned with the top of the next-printed swath so that a continuous image may be printed on the paper. Alternatively, the paper may be advanced by less that a full swath height to effect xe2x80x9cshinglingxe2x80x9d type of printing. In any event, inaccurate media advances between scans of the carriage result in print quality artifacts known as banding.
The prevention of banding artifacts thus calls for precise control of the advancing media in discrete steps between printed swaths. The demand for accuracy in advancing media becomes greater as printhead development leads to higher and higher resolutions, thereby reducing the tolerances permitted in advancing the media.
The speed with which the print media is moved through a printer is an important design consideration called xe2x80x9cthroughput.xe2x80x9d Throughput is usually measured in the number of sheets of printed media moved through the printer each minute. A high throughput is always desirable.
The time required for media advance between printed swaths is a large component of the overall time required for the printing task. Moreover, printhead development has and likely will provide increasingly large swath heights so that the media must be advanced a relatively larger distance between swaths, preferably without a reduction in throughput. Thus, the designer must balance the requirements for accurate media advance with the design goal of providing the highest throughput possible.
The tolerances permitted in media advance are so small that variations in system performance must be considered even within the same printer families, where otherwise identical drive motors and associated media-advance mechanisms are specified. For example, the friction characteristics of media-advance mechanisms (gears, feed rollers, etc.) in one printer will not precisely match those of another, otherwise identical printer. The same is true for the characteristics of the motor that drives the media-advance mechanisms. For convenience, these system frictions and motor characteristics will be hereafter collectively referred to as system response characteristics, which, as noted, vary at least to some degree from printer to printer.
The speed with which the printer is operated can exacerbate variations in system response characteristics. Thus, aggressively driving the media advance mechanisms to achieve the highest possible throughput would lead to, for example, banding artifacts in printers having relatively poor system response characteristics.
In the past, printer control systems have been designed to account for variations in system response characteristics so that all printers meet the media advance tolerances. One approach to this is to drive the media advance system conservatively so that acceleration and deceleration rates, as well as maximum velocities, can be achieved by worst-case systems (that is, systems with the poorest system response characteristics). It will be appreciated that this lowest-common-denominator approach inhibits the media-advance performance of systems that have average and above-average system response characteristics.
In other approaches, the conservative, worst-case drive approach is reserved for the end of the media advance step. That is, the media is advanced aggressively (rapidly) in a first stage for a majority of the incremental advance distance, but then slowed during a second (xe2x80x9cfinal approachxe2x80x9d) stage as the media moves into the proper position. Because of the large position errors that can arise during the first stage, the duration of the second stage is relatively long (despite the fact that the distance moved is small) in order to enable correction of the largest position errors. In this approach, therefore, throughput and media position accuracy is enhanced over what went before, although there remains room for improvement in throughput. The present invention provides that improvement.
The present invention is directed to a method of controlling a media-advance drive motor in a manner that preserves accuracy in the incremental advances of print media between printing swaths, while optimizing throughput and accounting for variations in printer system response characteristics.
The invention is primarily embodied in a printer control algorithm that commences each media advance step with a stage that accelerates the media-advance drive motor to its highest velocity. That is, the drive motor acceleration is not restricted to a predetermined acceleration curve, as may be provided with prior approaches to accommodate the worst-case systems as discussed above.
In the present invention, the printer""s processor requirements are reduced during this first stage since the applied motor voltage does not have to be computed during acceleration, as would be the case if the motor were driven to match a predetermined acceleration curve.
This initial acceleration stage of the algorithm eliminates performance penalties that would otherwise apply to systems that can accelerate faster than xe2x80x9cworst casexe2x80x9d systems. Also, the drive motor is operated more efficiently because it is quickly brought up to an efficient operating range, thereby resulting in a cooler operating temperature and longer motor life.
Upon completion of the acceleration stage, the drive voltage is reduced to zero so that the motor decelerates. During this deceleration stage, the motor velocity is monitored and the drive voltage is adjusted from zero as needed to conform to a predetermined decaying velocity versus position function that is representative of a specimen system. The velocity versus position function is correlated to the required media position so that a media-advance motor following that function will arrive at a zero velocity at the precise instant that the media arrives at the position corresponding to the end of its incremental advance.
In one preferred embodiment, reference is made to the predetermined velocity versus position function for the purpose of selecting the instant when the deceleration stage should begin. The velocity versus position function is recorded in the printer firmware as a look-up table or equivalent equation.
Inasmuch as the decelerating motor generally follows a natural deceleration curve, the servo control effort for keeping the motor velocity on that curve is minimal. Thus, the motor can readily transition to a fully stopped mode because there will be little applied voltage at that time, which, as noted, corresponds to the desired or target position of the media.
Other advantages and features of the present invention will become clear upon review of the following portions of this specification and the drawings.